1. Field of the Invention
The present invention is generally drawn to reactor tube rib plug removal systems and more particularly to such systems using TIG (Tungsten Inert Gas) welding techniques to relax the rib plug.
2. Description of the Prior Art
In tube-type heat exchangers, a first fluid flows through the tubes of the heat exchanger while a second fluid surrounds the outside of the tubes such that heat exchange occurs between the two fluids. One or more of the tubes can become defective over the life of the heat exchanger such that a leak occurs which allows the two fluids to mingle. When this occurs, it is necessary to plug the tube so that the fluid does not flow through the tube thereby preventing the leakage of the first fluid to the other fluid.
In nuclear reactor power plants, when such a defect occurs in the tubes of a nuclear steam generator the coolant in the tubes is allowed to mingle with the coolant outside of the tubes and a more significant problem arises. A radioactive contamination problem occurs. Since the fluid flowing in the tubes of a nuclear steam generator is generally radioactive, it is important that it not be allowed to leak from the tubes and contaminate the fluid surrounding the tubes. Therefore, when a leak occurs in a nuclear steam generator heat exchange tube, the heat exchange tube is plugged so that the coolant is not permitted to flow through the tube.
One type of tube plug used for the above application is described in U.S. Pat. No. 4,390,042, which patent is incorporated herein by reference thereto. This plug as best seen in FIG. 1 comprises a shell having a tapered inner surface against which an externally tapered expander member moves by application of a pulling force. The motion of the expander member relative to the shell causes the shell to expand into contact with a heat exchange tube thereby plugging the tube. The expander member is formed from a hardened metal with tangentially blended leading radius and a self-locking trailing edge, which provides for ease in expanding the shell while preventing inadvertent unlocking of the plug. The shell has a substantially uniform wall thickness throughout the portion of the shell which experiences expansion so that the force necessary to perform the expanding process does not increase beyond strength limitations of installation equipment as the expander member is moved through the shell. The shell also has a plurality of lands on the outside surface thereof whose depth increases from closed end to open end of the shell so that a substantially uniform wall thickness can be maintained in that region while allowing the lands to contact the inner surface of the heat exchange tube.
The described plug and its position sealing a faulty reactor stem generator tube are best seen in FIG. 2.
However, under certain circumstances these tube plugs may themselves leak either from a faulty installation or from wear over time and must be removed and replaced. Such removal must be done without damaging the reactor tube so a new plug may be easily installed.
Plug removal systems and methods for using same are well known. The systems utilize impact hammers and pullers and in some cases also use heaters.
The usual known methods involve the removal of the expander member that expands the rib plug to seal the pipe and the subsequent removal of the rib plug from the tube. These methods sometimes drill out the mandrel or expander and involve stretching and pulling the rib plug out of the pipe. Drilling is a very time consuming process and leaves behind numerous metal fragments that must be removed from the reactor tubes. Stretching and pulling the rib plug from the tube may be done by anchoring a member into the open end of the plug and using a push rod that continuously applies a force to the closed end of the plug to stretch it while removing the plug from the tube or by simply locking into the open end of the plug and removing it from of the tube. These removal methods are only occasionally successful and do significant damage to the inner diameter of the steam generator tube.
Other known methods push the mandrel into the rib plug bottom and then push the rib plug into the tube. These methods are well illustrated in the following patents:
U.S. Pat. No. 5,465,483 that teaches to hydraulically push the mandrel back into the end of the rib plug and then to use repetitive impulses to remove the rib plug from contacting the tube;
U.S. Pat. No. 5,517,740 which teaches to use repetitive impulses to displace the mandrel and use the same repetitive impulses to remove the plug from contacting the tube; and
U.S. Pat. No. 5,664,328 that teaches to hydraulically push the mandrel back and use repetitive impulses to remove the plug from contacting the tube. These methods while being cleaner than methods drilling out of the mandrel still damage the tube when the rib plug is pushed or pulled there from.
Other known methods add a heating step to condition the rib plug after disengaging the mandrel or expander and before removing the rib plug from the tube. These methods are well illustrated in the following patents:
U.S. Pat. No. 5,255,717 which teaches to hydraulically disengage the mandrel, use longitudinal welder or TIG passes along the inside surface of the rib plug, and then to remove the plug from the tube; and
U.S. Pat. No. 4,829,660 that teaches to hydraulically disengaging the mandrel, use longitudinal TIG passes along the inside surface of the rib plug, and to remove the plug from the tube.
These methods using longitudinal passes of the TIG welder has been only marginally successful in completely relaxing the rib plug so it may be pulled from the tube without some damage to the tube. Also, the removal of the rib plugs did not consistently provide an intact removal of the rib plug. In some cases the rib plug would break apart during removal causing parts to fall into the tube that required later pick up resulting in increased cost in time and money. Therefore a new method of rib plug removal was needed which would completely relax the rib plug from being presses into contact with the tube to allow a consistent removal of the entire rib plug intact and without scoring or otherwise damaging the tube.
The present invention is drawn to a method for removing Westinghouse type rib plugs from steam generator tubes. The method is a three-step process involving:
1)The removal of the expander element or mandrel from the expanded part of the rib plug. This may be done by either pushing the mandrel back into the end of the rib plug or by drilling it out or by impacting the mandrel with a slug driven by air;
2) The TIG welding of the internal surface of the plug in a spiral manner from the bottom to the top thereof to relax the rib plug from its forced contact with the tube; and
3) To lock on the relaxed tube plug open end and pull it out or to stretch and expand the plug as it is being pulled out.
In view of the foregoing it is seen that one aspect of the present invention is to provide a positive method of relaxing the rib plug from contact with the tube after the removal of the mandrel.
Another aspect of the present invention is to provide a method for removing rib plugs from tubes that will not score or deform the tube during the removal of the rib plug.
Yet another aspect of the present invention is to provide a method for consistently removing entire rib plugs from tubes without any breaking of the rib plug with a part falling into the tube.
These and other aspects will be more fully understood upon a review of the following description of the preferred embodiment when considered with the accompanying drawings.